Mobile communications devices have become an integral part of society over the last two decades. Indeed, more than eighty-two percent of Americans own a mobile communications device, for example, a cell phone. The typical mobile communications device includes an antenna, and a transceiver coupled to the antenna. The transceiver and the antenna cooperate to transmit and receive communications signals.
The typical transceiver includes a power amplifier for amplifying low voltage signals for transmission via the antenna. Given that most mobile communications devices operate on limited battery power, energy efficient power amplifiers may be desirable. More specifically and as will be appreciated by those skilled in the art, Class C and E power amplifiers are common in mobile communications devices since they are efficient power amplifiers. These classes of power amplifiers are more efficient than Class A or B amplifiers, for example, but are subject to performance tradeoffs. For example, they may be nonlinear over certain frequencies and may introduce greater amounts of distortion into the amplified signal (if the signal requires a linear amplifier).
An approach to compensating for this tradeoff is to encode transmitted signals with constant envelope (CE) or continuous phase modulation (CPM) waveforms. These modulations provide for lower energy demands on the power amplifier of the transceiver, for example, by reducing the peak to average power ratio (PAPR), increasing average transmit power (providing greater transmission range), and increasing amplifier efficiency.
For example, U.S. Patent Application Publication No. 2008/0039024 to Ikeda et al. discloses an amplifying circuit. The amplifying circuit processes an input orthogonal frequency-division multiplexing (OFDM) signal and provides a pair of CE output signals to provide better power efficiency and less distortion. A particularly advantageous approach is constant envelope orthogonal frequency-division multiplexing (CE-OFDM).
Another approach is Gaussian minimum shift keying (GMSK) modulations, which are part of the CPM waveform family. In a typical GMSK device, the digital data stream is first shaped with a Gaussian filter before being applied to a frequency modulator. This reduces sideband power, thereby reducing out-of-band interference between signal carriers in adjacent frequency channels.
A potential drawback to CPM and CE waveforms, for example, GMSK, may include use of complex memory based modulations at the transmitter wireless device. Use of such memory-based modulations typically increases the computational requirements at the receiver wireless device. More specifically, the typical memory based demodulator may include a maximum likelihood sequence estimator, which is computationally intensive and increases the required processing at the receiver wireless communications device. Further, the use of these complex computational devices is undesirable since they may reduce battery life.